This invention relates to a tool used during a refueling cycle of a nuclear reactor, and more specifically, to a device for applying tension to and removing tension from a vessel stud and nut combination which attaches a head of a reactor pressure vessel to the reactor vessel in a nuclear power plant.
The pressure vessel head is attached to a flange along an upper perimeter of the reactor vessel by a plurality of vessel studs with nuts. The studs are inserted through a hole in the vessel head and screwed into the vessel flange. A stud tensioner tool is used to compress a seal between the vessel head and the vessel flange. While seal compression is maintained, the nut on the vessel stud is rotated clockwise to tighten down on the stud so that the seal is retained when the stud tensioner tool is relaxed.
The vessel studs and attached nuts are removed from the reactor vessel at the beginning of a refueling cycle. The stud tensioner tool is again used to compress the seal between the vessel head and the vessel flange. The stud nut is then loosened a few turns so that the nut will remain loose after the stud tensioner tool is relaxed. Once all vessel studs have been detensioned, the studs and attached nuts are removed from the vessel flange. The vessel head may thereafter be lifted from the reactor vessel.
A stud tensioner tool comprises some means for engaging the top of the vessel stud above the nut plus means for applying a force to elongate the stud. "Elongation" refers to the lifting of the top of the vessel stud relative to the upper surface of the vessel head, the net effect being a compression of the seal between the reactor vessel flange and the vessel head. The force of elongation is commonly applied by a hydraulic cylinder assembly. The means for engaging the top of the vessel stud, however, has involved several arrangements.
Of the known prior art, the primary means for engaging the stud involves manually screwing the relevant portion of the tensioner tool onto the top of the vessel stud for approximately six turns. Once the stud is fully engaged and the elongation force is applied, the nut is automatically rotated in the appropriate direction by a nut turning device. When the nut has reached the desired position, the elongation force is relaxed and the tensioner tool is manually removed from the stud by an operator.
Engagement of the stud, when done by manual screwing, constitutes a significant percentage of the total time required to detension a stud. Time, during the stud tensioning, or detensioning, process, is important for two reasons. First, due to moderate radiation fields in the region of the studs, a significant dose accumulation of radiation is received by operating personnel during the stud detensioning process. The problem of radiation exposure is further complicated by the general trend of the Nuclear Regulatory Commission toward reduction of the allowable dosage of radiation to operating personnel.
The second reason that time is important arises from the significant cost to the utility when the reactor is down. Any decrease in the overall length of time required to refuel the reactor results in substantial financial savings to the utility. Improving the means by which the tensioner tool engages the vessel stud is thus a likely target for increasing refueling efficiency.
One prior art improvement seeks to minimize the amount of time spent screwing the tensioner onto and off of the stud by introducing a motorized screwing system. This improvement replaces the manual screwing motion of the operator with an automatic screwing system. While this improvement effectively increases the time efficiency of stud engagement, a concurrent increase in complexity of the tool results in more down time for maintenance and repair and a higher cost for tool production.
A second prior art improvement directed to the efficiency of the stud engagement process eliminates altogether the need for screwing the tool onto the stud. This is accomplished by breaking the portion of the tensioner having female threads into several sections. The sections may be drawn apart to increase the inner diameter of the stud engagement portion of the tool. This permits the tool to be inserted over the top of the stud, after which the sections are closed around the circumference of the vessel stud to effect engagement. This radial motion is accomplished by use of hydraulic cylinders.
The section-type engagement system operated by hydraulic cylinders is quite effective in reducing the time required for stud engagement. Its use, however, requires the addition of a second hydraulic system with its attendant need for periodic maintenance and repairs. The additional hydraulic system also increases the cost of the tool.
A third prior art approach to improving the stud tensioner tool utilizes a method of engaging the vessel stud known as roto-lock engagement. This improvement requires the use of a specially constructed stud. The threads on this stud, rather than being continuous around the circumference of the stud, are divided into three columns. The three columns of threads are separated by three columns of smooth stud surface without threads. The female threads of the stud engagement surface located inside the stud tensioner are constructed in a complimentary arrangement of three columns.
The stud is engaged by inserting the stud tensioner over the roto-lock stud so that the female threads of the stud tensioner are aligned with the smooth-surfaced columns on the vessel stud. Once the tensioner has been fully inserted over the stud, the tensioner is rotated approximately 60.degree. so that the columns of female threads within the stud tensioner engage the columns of male threads on the vessel stud.
The roto-lock engagement system, like other prior art improvements, successfully reduces the time required for stud engagement. The necessity of a specially constructed stud, however, reduces the desirability of this approach. Use of the roto-lock engagement system requires proper alignment of the columns of thread before insertion, and rotation of the tool to effect engagement. If these processes are done mechanically, the new apparatus contributes to maintenance time and tool cost. If the processes are done manually, the operator is subject to radiation exposure.
It would therefore be advantageous to construct an improved version of the stud tensioner tool, which version eliminates the manual threading system, thereby reducing the time required to engage and disengage the vessel stud and also reducing the radiation exposure of the operator. It would be advantageous if this improvement required no significant additions of hardware which would serve to increase the cost of construction of the tensioner tool and which would also serve to increase the time required to maintain and repair the tool. Finally, it would be advantageous to construct a stud tensioner which requires no special alignment during the engagement process and which is adaptable to existing vessel studs so that it may be used in current refueling operations.